The use of shape memory alloys for making certain components of metallic eyeglass frames has already been known.
Any metal alloy that is able to regain its initial shape by simple heating after having undergone a permanent deformation at a lower temperature is called a shape memory alloy (SMA).
This property is the macroscopic expression of a physical phenomenon involved in the atomic organization of the material. This transformation is known as the reversible martensitic transformation.
The martensitic transformation can be described, from a phenomenological viewpoint, by the curve shown in FIG. 1, in which the temperature is shown on the abscissa and the transformed fraction is shown on the ordinate.
On cooling (direct martensitic transformation), martensite begins to be formed beginning from the temperature Ms (martensite start), and the sample is completely martensitic at the temperature Mf (martensite finish).
On reheating (inverse transformation), austenitc (or .beta. phase) begins to be formed at a temperature As (austenitc start), and the sample is completely austenitic at the temperature Af (austenitc finish.
The reversible martensitic transformation is the cause of the specific properties of the shape memory alloys:
one-way memory effect, PA0 two-way memory effect, PA0 superelastic effect, PA0 rubber effect. PA0 Low fatigue strength of the Ti--Ni alloys, which displays a fragile rupture after some stress cycles. PA0 Work hardening may be used to palliate the poor fatigue strength of Ti--Ni alloys, but the superelastic deformation .epsilon..sub.max is limited to about 3% to 4% in this case. The rigidity of the Ti--Ni alloy increases rapidly beyond this limit, and the alloy tends to break. PA0 The Ti--Ni alloys contain a high percentage of nickel and must be coated with nickel for assembly by means of silver solder which is usually used in making eyeglasses. The element nickel may cause allergies in the wearer of the eyeglass frames. PA0 The assembly of the superelastic components made of Ti--Ni alloy with the components made of conventional metals commonly used in making eyeglasses (Monel, nickel silver, bronzes, euproberyllium, stainless steels, titanium alloys) cannot be easily performed. PA0 The first method consists of force-fitting the end or ends of the superelastic component into a socket made of a metal that can be soldered with silver. With the superelastic component being fitted into the socket, the socket is then soldered to the conventional components. PA0 The second method consists of coating the superelastic component with a galvanic nickel layer and subsequently soldering to the conventional component with a silver solder. PA0 The third method consists of welding the superelastic components to the conventional components with the energy of a laser beam or electron beam. These expensive methods are, however, rarely employed in making eyeglasses. PA0 Cu--Al--Cu, PA0 Cu--Al--Ni, PA0 Cu--Al--Be, PA0 Cu--Al--Mn, PA0 Fe--Mn--Si, PA0 Fe--Mn--Cr, PA0 Fe--Mn--Cr--Si.
The martensitic transformation takes place not only in the case of T&lt;Ms, but also at T&gt;Ms by the application of a mechanical stress. If this stress is released for an alloy deformed at T&gt;Af, inverse transformation of the martensite into austenitc takes place with a transformation hysteresis. This recoverable phenomenon is called the superelastic effect. It is illustrated in FIG. 2, in which the deformation is shown on the abscissa and the mechanical stress is shown on the ordinate.
The deformation .epsilon..sub.max is the maximum deformation .epsilon. which a shape memory alloy is able to withstand without irreversible plastic deformation persisting after the shape memory alloy is relieved of any mechanical stress.
The standard NF A 51-080 defines the terminology and the measurements associated with the shape memory alloys. The terms used in this text refer to that standard. The standard can be obtained from AFNOR (Association Francaise de Noralisation), Tour Europe, Cedex 7, 92080 Paris La Defense, France.
There currently are eyeglass frames containing temples, bridges or bars in which all or part of these components are made of a superelastic shape memory alloy. The shape memory alloy used to prepare these components is an alloy based on titanium and nickel. This results in the following considerable difficulties, inter alia, in terms of the preparation and the mechanical strength of the eyeglass frames:
Three methods are currently employed to assemble the superelastic components with the conventional components (which lack the property of superelasticity):
It should be noted that a galvanic nickel coating may be used to improve the coupling and the soldering.
Other shape memory alloys have also been tested for making superelastic eyeglass frame components. These tests and the patents that may have resulted therefrom mention Cu--Zn--X alloys (in which X=Al, Sn, . . . ) and Cu--Al--Ni. However, compared with the alloys based on Ti--Ni, these alloys cannot withstand .epsilon..sub.max deformations greater than 3% to 4%. Beyond this limit, they are fragile and break because of their coarse grain structure.
It is currently desired to improve the mechanical strength and the use of the shape memory alloys used in making eyeglasses, such as Ti--Ni as well as other shape memory alloys.
The methods used consist of hardening the material in order to increase its yield strength as much as possible.
A very fine-grain structure is desirable for this purpose; it is obtained either by recrystallization of the coarse structures generated in the foundry after considerable work hardening, or by structural hardening achieved in the course of heat treatments causing the precipitation of the hard phases, or by cold working.
The major drawback of these methods is that the deformations associated with superelasticity are greatly limited. The grain boundaries, the precipitates and the work hardening block the martensitic transformation and lead to a great increase in the rigidity of the superelastic alloy.